This invention relates to the purification of an atmosphere of pollutants. More specifically, the invention relates to the purification of an atmosphere containing carbon monoxide. Still more specifically, the invention relates to a method of purifying an atmosphere of carbon monoxide by catalytic oxidation of the carbon monoxide to carbon dioxide.
Carbon monoxide (CO) is the most widely distributed and the most commonly occurring air pollutant. Total emissions of CO to the atmosphere exceed those of all other pollutants combined. Most atmospheric CO is produced by the incomplete combustion of carbonaceous material used as fuels for vehicles, space heating, and industrial processing, or burned as refuse. Automobiles are especially notorious for producing this gas. It has been estimated that in Los Angeles County more than 3 million motor vehicles per day pollute the air with 8000 tons of carbon monoxide, for an average of more than 5 pounds per vehicle per day.
Carbon monoxide is an odorless and colorless gas which is a poisonous inhalant. No other toxic gaseous air pollutant is found at such relatively high concentrations in the urban atmosphere. Carbon monoxide is dangerous because the affinity of hemoglobin is about 200 times greater for carbon monoxide than it is for oxygen. Thus, the effect of carbon monoxide on the human body is to deprive the tissues of necessary oxygen. 100 parts per million is generally considered the upper limit of safety in industry for healthy persons within certain age ranges when exposure may continue for an 8-hour period. Many people experience dizziness, headaches, lassitude and other symptoms at approximately 100 ppm.
Present measurements do not show that this level is often exceeded in cities in the United States, although in commercial and industrial districts of certain cities the concentration of carbon monoxide has ranged from 0 to 55 ppm, with an average of 9.5 ppm. Concentrations higher than this occasionally occur in garages, tunnels, behind automobiles and in the open atmosphere.
Since carbon monoxide results from incomplete combustion of carbonaceous materials, the obvious control measure is more complete combustion. In many cases this will lead to increased fuel economy. Combustion of carbon monoxide yields nearly as much heat as the combustion of fuels to carbon monoxide. Today's automobile engines do not function well on fuel mixtures sufficiently lean to accomplish a major reduction of the carbon monoxide emitted. The present approach to automobile exhaust control is to use catalysts which will oxidize the poisonous carbon monoxide to carbon dioxide which is relatively harmless. A number of catalysts are known which produce this oxidation reaction, such as copper oxide-alumina catalyst, platinum-nickel, rhenium catalyst and copper-chromium catalyst. The catalyst which appears to show the best promise in the catalysis of this reaction is a 1/2% platinum on an inert substrate material catalyst. However, there are problems associated with the use of a platinum catalyst, principally in its cost, which at present is approximately $2000 per pound.